1. Field of the Invention
The invention relates to grinding media and more particularly to zirconium silicate grinding media.
2. Description of the Prior Art
Many applications such as the production of ceramic parts, production of magnetic media and manufacture of paints require that the ceramic, magnetic or pigment powder, respectively, be as completely dispersed within the particular binder appropriate for a given application as possible. Highly dispersed ceramic powders result in ceramic parts of higher density and higher strength than those prepared from less completely dispersed solids. The data storage capabilities of magnetic media are limited by particle size and completely dispersed, finely divided powder magnetic media achieve maximum information storage. The optical properties of paints, such as hiding power, brightness, color and durability are strongly dependent on the degree of pigment dispersal achieved. Finely divided powders are required to achieve such complete powder dispersal. Typically, milling devices such as disc mills, cage mills, and/or attrition mills are used with a milling medium to produce such finely divided powders, ideally to reduce the powder to its ultimate state of division such as, for example, to the size of a single powder crystallite.
Milling of some powders involves a de-agglomeration process according to which chemical bonds, such as hydrogen-bonded surface moisture, Van der Waals and electrostatic forces, such as between particles, as well as any other bonds which are keeping the particles together, must be broken and/or overcome in order to obtain particles in their state of ultimate division. One pigment powder which entails a de-agglomeration milling process to reduce it to a finely divided powder is titanium dioxide. Optimal dispersal of titanium dioxide pigment powder results in optimized performance properties, particularly improved gloss, durability and hiding power.
De-agglomeration processes are best performed using a grinding medium characterized by a small particle size which is the smallest multiple of the actual size of the product particles being milled which can still be effectively separated from the product powder. In a continuous process, the grinding medium can be separated from the product particles using density separation techniques. In a typical bead or sand mill operated in a continuous process, separation of the grinding medium from the product can be effected on the basis of differences between settling rate, particle size or both parameters existing between the grinding medium and product powder particles.
Commercial milling applications typically use silica sand, glass beads, ceramic media or steel balls, for example, as grinding media. Among these, the low density of about 2.6 g/cc, of sand and glass beads and the low hardness of glass beads restricts the materials which can be milled using sand or glass beads. The use of steel shot is restricted only to those applications where iron contamination resulting from wear products of the steel shot during the milling process can be tolerated.
Thus, there exists a need for a relatively inexpensive, dense and non-toxic grinding medium which is characterized by a small particle size, a density sufficiently high for separation purposes to allow it to be used for the milling of a wide range of materials and which does not generate wear byproducts which result in contamination of the product powder.